How do I install from unknown sources?

‎03-13-201306:41 PM

My 18382GM Tablet failed to boot recently and was sent back to Lenovo for repair. They reimaged it and now I am trying to install the apps I had on it previously. One app is not on the Play store or App store. I don't see an "unknown sources" option under "Settings" > "Apps" to allow me to intall it with the apk file.